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Reclaimed Asphalt Pavement 153 SELECT 8 -10 RANDOM SAMPLING LOCATIONS OBTAIN STOCKPILE SAMPLES AASHTO T2 Size = 10 kg x (number of mix designs) + 5 kg 5 kg SPLIT REMAINDER SAMPLE AASHTO T248 COMBINE WITH OTHER 2.5 kg SPLIT 2.5 kg LOCATIONS TO CREATE SAMPLE REPRESENTATIVE SAMPLE AASHTO T248 COMBINE WITH OTHER BINDER CONTENT AND LOCATIONS TO CREATE GRADATION FOR REPRESENTATIVE SAMPLE BLENDING AND VARIABILITY ANALYSIS 2.5 kg SPLIT REMAINDER SAMPLE AASHTO T248 CHARACTERIZE RAP CHARACTERIZE BINDER PROPERTIES RAP AGGREGATE IF REQUIRED PROPERTIES VOLUMETRIC MIXTURE DESIGN AND PERFORMANCE ANALYSIS Figure 9-2. Flow chart for recommended sampling of RAP stockpiles. prepared. Figure 9-2 is a general flowchart for sampling a RAP stockpile for use in HMA mix- ture design. Blending and Variability RAP variability is an important consideration in the design of HMA incorporating RAP. Many agency specifications require mixtures with RAP to be produced to the same production tolerances as mixtures made with all new materials. If highly variable RAP is used, then the HMA may not meet production tolerances, resulting in lost production time, a penalty, or, in extreme cases, the need to remove and replace the mixture. The amount of RAP that can be added without exceeding
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154 A Manual for Design of Hot Mix Asphalt with Commentary specification limits depends on the limits themselves, the variability of the RAP, the variability of similar mixtures produced without RAP, and the consistency of the equipment adding the RAP. Unfortunately, the calculation of maximum RAP content based on a variability analysis is quite complicated and often some of the information required is uncertain or even unknown. For these reasons, the approach recommended in this manual is somewhat simplified, but based soundly on the basic statistical theory involved. It will provide engineers and technicians with reasonable estimates of the amount of RAP that can be incorporated into a mix design without unaccept- able increases in production variability. Two different methods can be used to determine the maximum allowable RAP content based on variability: a graphical approach and the HMA Tools spreadsheet. The same statistics are used in both cases, but HMA Tools is a more precise approach that will often allow for somewhat higher RAP contents. Because many state highway agencies already have specifications in place establishing allow- able RAP contents in HMA mixtures, HMA Tools allows any RAP content in the design process. If desired, HMA Tools can be used to perform the statistical analysis of RAP stockpiles and to determine the maximum allowable RAP content. However, this is not required. Thus, the user can enter any desired RAP content when using HMA Tools to develop mix designs. RAP Binder Content and Aggregate Gradation: Laboratory Procedures The average and standard deviation of the binder content and aggregate gradation in the RAP stockpiles are properties that must be measured to effectively design HMA with RAP. The 5 kg sub-samples split from the sample taken at each sampling location are used for this analysis. If reasonable estimates of the ignition oven correction factors for local aggregates can be made, then the RAP binder content can be determined using an ignition oven, AASHTO T 308. The gradation of the RAP aggregates is then determined using AASHTO T 30 after application of the aggregate correction factors as described in AASHTO T 308. If correction factors for local aggregates are unknown or highly variable, then the RAP binder content must be determined by solvent extraction, AASHTO T 164. The gradation of the extracted aggregate is then determined using AASHTO T 30. If desired, ignition oven correction factors can be established by performing both analyses on split samples from at least three locations in the stockpile. Determining Combined Gradation and Binder Content The computation of blends for mixtures incorporating RAP is a little different than that for mixtures made with all new stockpiles. When RAP is used, the RAP material that is added includes both the RAP aggregate and the RAP binder. Since gradation data are based on the weight of aggregate, and binder contents are based on the total weight, the stockpile percentages must be adjusted for combined gradation analysis based on the amount of binder contained in the RAP. In using HMA Tools to determine the composition of HMA mixes containing RAP, data on the aggregate gradations--including the gradation of aggregate contained in the RAP-- is entered in the worksheet "RAP_Aggregates." Aggregate bulk and apparent specific gravity data is also entered here, along with the asphalt binder content and specific gravity. Data for up to four RAP stockpiles can be entered. General information for the mix--most importantly the tar- get VMA and air void content--are entered in the worksheet "General." The actual composition of the blend is entered in the worksheet "Trial_Blends," which then lists the combined gradation and various other data for the mix. The example below illustrates the use of HMA Tools in cal- culating the composition of an HMA mixture containing RAP.
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Reclaimed Asphalt Pavement 155 Example Problem 9-1. Gradation and Binder Content Analysis for an HMA Mixture Containing RAP A 9.5-mm NMAS mixture will be produced using three new aggregate stockpiles, hydrated lime, and two RAP stockpiles. Gradation and binder content data for each stockpile is given in Table 9-1. The stockpiles will be combined in the follow- ing proportions: 29% of Aggregate 1, 40% of Aggregate 2, 10% of Aggregate 3, 1% lime, 10% Coarse RAP, and 10% Fine RAP. The target VMA should be 16.0% and the target air void content 4.0%. The specific gravity value for both the new binder and the RAP binders is 1.030. For this combination of stockpiles, compute the combined gradation, the binder provided by the RAP, and the amount of new binder required. Table 9-1. Stockpile materials for example 1. Property Sieve Size, Agg. 1 Agg. 2 Agg. 3 Hydrated Coarse Fine mm Lime RAP RAP 19.0 100 100 100 100 100 100 12.5 100 100 100 100 100 100 9.5 91 100 100 100 94 100 Gradation, 4.75 19 98 90 100 34 91 % Passing 2.36 6 61 52 100 25 65 1.18 5 37 31 100 22 46 0.600 4 24 20 100 20 34 0.300 4 16 14 100 16 25 0.150 3 8 10 96 12 19 0.075 2.9 3.6 8.4 89 10.4 15.7 Agg. Bulk Spec. Grav. 2.610 2.627 2.619 2.602 2.624 2.614 Agg. App. Spec. Grav. 2.628 2.651 2.645 2.675 2.638 2.637 Binder Content, % --- --- --- --- 3.1 4.5 Solution The general mix information is entered in the worksheet Table 9-2. "General"; this must include the target VMA of 16% and Combined the target air void content of 4%. The gradation informa- gradation for the tion and specific gravity values for the aggregates are proposed HMA entered in the worksheet "Aggregates" and for the RAP mixture for in the worksheet "RAP_Aggregates." Asphalt binder con- example 1. tent and other information for the RAP is also entered Sieve Size, Percent in this worksheet. The composition of the blend, that is, mm Passing the weight percentage of each aggregate and the RAP 19 100 materials, is entered in the worksheet "Trial_Blends" as 12.5 100 9.5 97 Trial No. 1. Make sure to enter the target VMA and tar- 4.75 67 get air void content in this worksheet. The total binder 2.36 41 content for the mix is given in cell F43 as 5.46%. Of this 1.18 27 0.60 19 amount, 0.75% is from the RAP (cell F143) and 4.72% 0.30 14 from the new binder (cell F145). The combined gradation 0.15 9 for this example problem is given in Table 9-2. 0.075 6.6
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156 A Manual for Design of Hot Mix Asphalt with Commentary Limiting Variability in HMA Mixes Containing RAP Because the variability of the RAP properties in a given stockpile can be quite large, it is important to estimate this variability and make sure that the addition of the RAP to an HMA mixture will not cause unacceptable increases in production variability. Controlling variability in mixes containing RAP involves three steps: 1. Sampling RAP stockpiles, as described previously. 2. Calculating the standard deviation for aggregate percent passing and binder content for all RAP stockpiles. 3. Estimating the maximum amount of RAP that can be added to the mix without exceeding allowable production variability. The variability of a mixture of several components, such as HMA, depends on the variability of the components, the proportions of the components, the precision of the blending, and the mean value of the components. Calculation of the standard deviation for aggregate gradations (percent passing) and asphalt content for HMA containing RAP can be quite complicated; as with other aspects of developing RAP mix designs, the details are not presented here but are included in the Commentary. The mean and standard deviation values for aggregate gradation and asphalt binder content are calculated in HMA Tools in the worksheet "RAP_Variability." Data for aggre- gate gradation and binder content are entered here for up to four RAP stockpiles. Data for up to 30 specimens can be entered for each of these four stockpiles. If no more than 15% RAP is to be used in a mix design, there is no need to perform a variability analysis of the RAP stockpiles used in the mix design. The standard deviation values calculated in the worksheet "RAP_Variability" are only esti- mates of the true values. There is a 50% chance that the true standard deviation for a given RAP stockpile will be higher than the calculated value. There is a relatively small chance that the true standard deviation will be much, much higher than the estimated value. Because the standard deviation values calculated for the RAP stockpiles are only estimates, the values used by HMA Tools in determining the maximum allowable RAP content is an upper confidence limit, rather than the calculated value. In cell B6, the reliability (confidence) level for this esti- mate is entered; a value of 80% is suggested. The more samples used in calculating the stan- dard deviation, the more accurate the estimate will be and the lower the value of upper confi- dence limit for the standard deviation. Therefore, it is suggested that at least five samples be used for calculating the mean and standard deviation for a RAP stockpile. Larger numbers of samples--up to 30--will provide greater accuracy and will normally allow greater percentages of RAP to be used in the mix design. In using HMA Tools to perform a variability analysis of RAP stockpiles, gradation data for up to 30 samples for the first RAP stockpile are entered in cells B19:AE31; asphalt content is entered in cells B33:AE33. Gradation data for up to three more RAP stockpiles are entered in cell ranges immediately below this. The approximate proportions to which the stockpiles will be blended must be entered in cells B9:B12. The estimated maximum allowable RAP content will appear in cell B14 when data entry is complete and the calculation is completed. Because up to 15% RAP can be used in any HMA design without performing a variability analysis, the maximum allowable RAP content will never go below 15%. Similarly, since handling large amounts of RAP during HMA production is often difficult for practical reasons, HMA Tools limits the maximum allowable RAP to 50%. If needed, the average percent passing and stan- dard deviation for percent passing for the first RAP stockpile can be read in cells AH19:AH31 and AI19:AI31, respectively. The average and standard deviation of the asphalt binder content
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Example Problem 9-2. Calculation of Mean, Standard Deviation, and Maximum Allowable RAP Content for a Single RAP Stockpile A 30,000-ton RAP stockpile was constructed using millings from several projects. Table 9-3 summarizes the results of binder content and gradation tests on 10 ran- dom samples from the stockpile. Calculate the mean, the standard deviation, and the 80% upper confidence limit of the standard deviation for percent passing. Also, using HMA Tools determine the maximum allowable RAP content for this RAP stockpile. Table 9-3. Results of binder content and gradation tests on RAP stockpile for example 2. Sieve Sample Number Property Size, 1 2 3 4 5 6 7 8 9 10 mm 19.0 100 100 100 100 100 100 100 100 100 100 12.5 98 100 100 99 99 100 100 100 98 98 9.5 91 98 100 94 97 97 95 93 94 94 4.75 67 77 75 71 73 78 75 69 70 72 Gradation, 2.36 53 59 55 54 58 59 57 50 52 53 % Passing 1.18 39 44 48 43 49 46 45 41 39 41 0.600 32 38 37 35 39 36 37 33 32 33 0.300 22 27 25 23 26 23 25 21 22 22 0.150 14 17 16 15 16 14 15 14 13 15 0.075 10.7 12.2 11.9 10.7 12.9 10.3 11.9 10.5 9.8 10.8 Asphalt Content, % 4.0 4.5 4.7 4.4 5.1 4.6 4.6 4.3 4.6 4.8 Solution The values for percent passing and asphalt binder content given in Table 9-3 are entered in cells B19:K31 and B33:K33 in the worksheet "RAP_Variability." The reliability level in cell B6 should be the default value of 80%. Only one RAP stock- pile is being used, so 100 is entered in cell B9, and cells B10:B12 are left blank. After calculation (press F9 to make sure HMA_Tools performs the needed calcula- tions), the mean values are given in cells AH19:AH31 and AH33, the standard deviation values are given in cells AI19:AI31 and AI33, and the values for the upper confidence limit for standard deviation are given in cells AJ19:AJ31 and AJ33. Values for average, standard deviation, and the upper confidence limit for standard deviation are listed in Table 9-4. The maximum allowable RAP content of 42% appears in cell B19. Table 9-4. Computed averages and standard deviations for the RAP stockpile for example 2. Standard Upper Confidence Property Sieve Size, Average Deviation Limit for Std. Dev. mm 19.0 100.9 0.00 0.00 12.5 99.2 0.92 1.19 9.5 95.3 2.67 3.45 4.75 72.7 3.56 4.61 Gradation, % 2.36 55.0 3.13 4.04 Passing: 1.18 43.5 3.54 4.57 0.600 35.2 2.57 3.33 0.300 23.6 2.01 2.60 0.150 14.9 1.20 1.55 0.075 11.2 0.99 1.28 Asphalt Binder --- 4.56 0.295 0.382 Content, Wt. %
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158 A Manual for Design of Hot Mix Asphalt with Commentary appear in cells AH33 and AI33, respectively. The average and standard deviation of the per- cent passing and binder content for up to three additional RAP stockpiles appear immediately below these cells. Determining Maximum Allowable RAP Content Based on Variability Using a Graphical Approach Instead of using HMA Tools, a graphical approach can be used to determine the maximum allowable RAP content in an HMA mix design. Figures 9-3 through 9-6 are design charts for estimating the maximum allowable RAP content for an HMA mix design, based on the vari- ability in gradation and asphalt binder content of the RAP. Figure 9-3 gives the maximum RAP content based on the standard deviation for aggregate percent passing for a single RAP stock- pile. Figure 9-4 gives estimated maximum RAP content based on the standard deviation for asphalt binder content for a single RAP stockpile. Figure 9-5 gives estimated maximum RAP content based on the average standard deviation for aggregate percent passing for a blend of RAP stockpiles, while Figure 9-6 gives estimated maximum RAP content based on the average Standard Deviation for RAP Aggregate % Passing 0 1 2 3 4 5 6 7 8 9 10 50 Max. RAP Content, Wt. % 45 40 35 30 25 20 15 Sieve size, mm: 0.075 0.150 0.300 1.18 4.75 > 9.5 & 0.600 & 2.36 & 9.5 Figure 9-3. Maximum RAP content as a function of standard deviation for aggregate percent passing. For n = 5 Samples from a single RAP stockpile. 50 Max. RAP Content, Wt. % 45 40 35 30 25 20 15 0.2 0.3 0.4 0.5 0.6 0.7 Binder Standard Deviation Figure 9-4. Maximum RAP content as a function of standard deviation for asphalt binder content. For n = 5 Samples from a single RAP stockpile.
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Reclaimed Asphalt Pavement 159 Average Standard Deviation for RAP Aggregate % Passing 0 1 2 3 4 5 6 7 8 9 10 50 45 Max. RAP Content, Wt. % 40 35 30 25 20 15 Sieve size, mm: 0.075 0.150 0.300 1.18 4.75 > 9.5 & 0.600 & 2.36 & 9.5 Figure 9-5. Maximum RAP content as a function of average standard deviation for aggregate percent passing. For n = 5 Samples from a blend of RAP stockpiles, and no stockpile making up more than 70% of the RAP blend. 50 Max. RAP Content, Wt. % 45 40 35 30 25 20 15 0.2 0.3 0.4 0.5 0.6 0.7 Average Binder Standard Deviation Figure 9-6. Maximum RAP content as a function of average standard deviation for asphalt binder content. For n = 5 Samples from a blend of RAP stockpiles, and no stockpile making up more than 70% of the RAP blend. standard deviation for asphalt binder content for a blend of RAP stockpiles. Figures 9-5 and 9-6 are different from 9-3 and 9-4 because, when several RAP stockpiles are blended, the variabil- ity in the resulting blend will tend to be significantly lower than the variability in the individ- ual stockpiles. Figures 9-5 and 9-6 are based on the assumption that no RAP stockpile in the RAP blend will make up more than 70% of the RAP blend; if this assumption is not correct, Fig- ures 9-3 and 9-4 should be used with the standard deviation values for the stockpile making up most of the RAP blend. All four charts are based on statistics calculated from five independent samples; they cannot be used for smaller sample sizes. These charts can be used for statistics cal- culated using more than five samples, but doing so will tend to underestimate the amount of RAP that can be used in the mix design. To use these charts, the maximum allowable RAP must be determined for each sieve size for which the percent passing is less than 100%. The maximum allowable RAP must also be determined for the asphalt binder content. The maximum allowable RAP content is then the lowest of all these individual values. The procedure is probably best illustrated with an example problem.
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160 A Manual for Design of Hot Mix Asphalt with Commentary Example Problem 9-3. Determination of Maximum Allowable RAP Content Based on Variability Analysis Using the Graphical Approach Using the standard deviation values for aggregate percent passing and asphalt binder content calculated for Example 2, estimate the maximum allowable RAP content based on variability using Figures 9-3 and 9-4. Solution Table 9-5 lists the average percent passing and standard deviation for percent pass- ing for the RAP stockpile first introduced in Example 2. This also shows the average and standard deviation for asphalt binder content. The last column in Table 9-5 shows the maximum allowable RAP content based on variability for each individual sieve and asphalt binder content, as determined using Figures 9-3 and 9-4. The val- ues range from 30 to 50%; the lowest value is 30% (for the 1.18-mm sieve and the asphalt binder content); therefore, the overall maximum allowable RAP content based on variability is 30%. Note that this percentage is significantly less than the 42% found in Example 2 for the same standard deviation values. The value deter- mined using the graphical approach is lower because it is based on a sample size of n = 5. As mentioned above, using Figures 9-3 through 9-6 for cases where samples sizes larger than 5 are used to calculate standard deviation values will provide lower estimates of maximum allowable RAP content than would be found using more accurate methods such as the HMA Tools spreadsheet. Table 9-5. Standard deviation values and estimated maximum allowable RAP content for example 3. Maximum Standard Allowable RAP Property Sieve Size, Average Deviation Content mm % 19.0 100.9 0.00 50 12.5 99.2 0.92 50 9.5 95.3 2.67 50 4.75 72.7 3.56 40 Gradation, % 2.36 55.0 3.13 38 Passing: 1.18 43.5 3.54 30 0.600 35.2 2.57 38 0.300 23.6 2.01 50 0.150 14.9 1.20 50 0.075 11.2 0.99 50 Asphalt Binder Content, Wt. % --- 4.56 0.295 30 Maximum Allowable RAP Content for Stockpile: 30 Maximum RAP Content, Variability and Binder Properties The methods described above for estimating maximum allowable RAP content are based only on variability analysis--the maximum values for RAP content determined in this way only provide an estimate of how much RAP can be used in a mix design without significantly increasing production variability. These maximum values do not address the equally important issue of how the RAP content will affect the final binder grade in the HMA mix. The asphalt binder contained